No, you can not convert a Ruger Vaquero .45 colt to shot a .454 Casull. The frame of the gun will not withstand the pressure that is created with .454 Casull round. Also the frame of the gun with not let you load a round due to the length of the .454 cartridge. A Ruger SuperRed Hawk .454 Casull would be your best option, if you want to own a .454 Casull.
First off the Ruger .45 Colt “old” Vaquero is a stout firearm, but it can’t handle the pressure of the .454 or Ruger would be selling a .454 Cassull Blackhawk. Now your problem is saying bullets. The .454 Cassull and .45 Colt both use the same size bullets. So yes a .45 Colt could in fact safely fire .454 Casull bullets, but never a .454 Casull cartridge.